Referring to FIG. 1, a vehicle drive train having an unsynchronized transmission pointing in the forward direction as designated by the arrow 10 as shown. The drive train 20 includes an engine 22, a transmission 24, a clutch assembly 26 mechanically positioned between the engine and transmission, an axle 28 and a drive shaft 30 mechanically positioned between the transmission 24 output and the axle 28. In addition, a clutch pedal 30 and a gear shift lever 32 located in the driver's cab are mechanically connected to (not shown) and adapted to control the clutch assembly 26 and transmission 24 respectively in a well known manner.
Engine 22 includes a crankshaft 34 which can be considered to extend into the clutch assembly 26. A first disc 35 permanently attached to the crank shaft is mechanically positioned in the forward portion of the clutch assembly 26. The transmission 24 includes a input shaft 36 which extends from the rear into the clutch assembly 26. A second disc 37 is permanently attached to the input shaft and is mechanically positioned in the rearward portion of the clutch assembly 26, opposed the first disc 35. The clutch pedal 30 allows the driver to mechanically control the relative position of the discs 35 and 37 with respect to one another. When the clutch pedal 30 is in the up position, the clutch assembly 26 physically engages the two discs together, and the crankshaft 34 drives the input shaft 36 at the engine speed. When the driver pushes the clutch pedal 30 to the floor of the cab to disengage the clutch, the clutch assembly acts to physically separate the discs. The front disc 35 will rotate at engine 22 rpm, while the rearward disc 37 will rotate at the same speed as the input shaft 36.
To re-engage the clutch, it is mechanically necessary for the two shafts to be rotating at substantially equal rates to effectuate a smooth coupling between the two discs 35 and 37, respectively. This may require the driver to rev (or slow down) the engine speed so that the engine shaft rpm matches the transmission shaft rpm. This procedure is difficult, however, because the transmission shaft rate at the uncoupling is unknown because the single needle engine tachometer of the prior art no longer indicates the transmission rpm. The driver is required to rely solely on his experience at matching the engine speed with the transmission speed. Accordingly, a dual needle tachometer for indicating the engine and the transmission input shaft rpms on a single display face is needed to aid clutch engagement when the transmission is in gear.
Another problem confronting drivers is to effectuate smooth gear meshing for a vehicle coasting in neutral, based on the vehicle's road speed. By way of background, the operation of truck transmissions is provided. The transmission 24 is an unsynchronized gear box comprising an assembly of the transmission input shaft 36 and a gear train 38 comprising a multiplicity of paired gears mechanically positioned in the gear box. Each gear pair includes a first gear driven by the input shaft 36 and a second gear driven by the rear road wheels, through the drive axle and drive shaft 30, behind the transmission. When the driver places gear shift lever 32 into a particular gear position and engages the clutch, the first gear of the selected gear pair, mechanically coupled to the input shaft 36 rotates at the input shaft 36 rpm. The second gear of the selected gear pair, mechanically coupled to the drive shaft 30, maintains an rpm proportional to the vehicle's road speed. To effectuate vehicle movement, the gear pair must be meshed together to mechanically link the input shaft 36 driven by the crankshaft 34 to the drive shaft 30 by a meshing of the selected gear pair. The two gears must be rotating at substantially the same tangential speed to effectuate a smooth meshing of the mated gear pair. In other words, the driver is required to adjust the engine rpm to cause the particular gear pair to run at "synchronized" speed. If the engine speed is not effectively adjusted, the paired gears grind when meshing, or cannot be meshed at all.
Accordingly, shifting gears in a vehicle having an unsynchronized transmission includes the following steps:
(1) disengaging the clutch, thereby physically separating the mechanical link between the crankshaft 34 and transmission input shaft 36; PA1 (2) shifting the transmission 24 into neutral, to unmesh the gear pair; PA1 (3) engaging the clutch, thereby driving the input shaft and the first gear at rate equal to the engine rpm; PA1 (4) adjusting the engine rpm (i.e., revs up the engine for downshifting, or let the engine speed coast down if upshifting) so that the pair of gears to be meshed rotate at substantially their synchronized speed PA1 (5) disengaging the clutch; PA1 (6) shifting the transmission into gear to effectuate smooth gear meshing; and PA1 (7) engaging the clutch.
This procedure is difficult because the drive shaft rpm after clutch disengagement is unknown. The prior art single needle engine speed tachometer indicates only engine rpm and not drive shaft 30 rpm. The driver is required to rely solely on his experience in adjusting the engine speed which is indicated on the single needle tachometer, to effect meshing of transmission gears.
The prior art, as described above, has additional limitations. A mismatch in engine and transmission speeds due to an inaccurate estimation of drive shaft speed will cause the transmission gears to be unmeshable. Furthermore, repeated inaccurate estimations of the required engine rpm can lead to a host of long term mechanical difficulties. Namely, it decreases clutch life and causes damage to the transmission and drive shafts. When the driver fails to achieve transmission gear meshing, the vehicle can coast only, and motive force as well as engine braking is unavailable. A dual needle tachometer having a first needle for indicating engine rpm and a second needle for indicating transmission input shaft speed will aid drivers in matching engine and transmission speeds to facilitate smooth clutch engagement.